The present invention relates to a disk array apparatus which realizes a single high-reliability, high-performance magnetic disk unit (to be referred to as a logic device hereinafter) by causing a plurality of small, inexpensive magnetic disk units (each individual magnetic disk unit will be referred to as a physical device hereinafter) to perform synchronous parallel processing.
As is well known, "A Case for Redundant Arrays of Inexpensive Disks (RAID)," Technical Report UCB/CSD 87/391, Dec. 1987 has reported RAID1 to RAID5 as forms of a disk array apparatus.
In one of the disk array apparatus control schemes proposed in this technical report, in which a disk array apparatus uses a data parity, if an unrecoverable read error occurs in one physical device, the data of a physical sector in which the read error has occurred is repaired by using the data of other physical devices. The repaired data is then transferred to a host apparatus. Upon receiving from the host apparatus (host computer) a designation of reallocation of a replacement sector to the physical sector in which the unrecoverable read error has taken place, the disk array apparatus separates the physical sector with the read error and writes the repaired data in the replacement sector reallocated by the host apparatus.
As described above, if an unrecoverable read error takes place in one physical device of a conventional disk array apparatus, the data of a physical sector in which the read error has occurred is repaired using the data of other physical devices, and the repaired data is transferred to a host apparatus.
In this control scheme, however, if a physical sector of a physical device in which a read error has occurred is left unrepaired by a host apparatus, a decisive data fault is caused by occurrence of a fault at the same address in another physical device. Additionally, the load on the host apparatus is increased if the host apparatus is to perform separation of an error sector and rewrite of data. Therefore, if an unrecoverable read error takes place in a physical device, it is necessary to apply an extra load on the host apparatus to maintain high reliability.
As another prior art, Japanese Patent Laid-Open No. 62-285162 (to be referred to as "prior art" hereinafter) discloses a "memory device" in which a substitute memory area is added to a shared memory device. A CPU of this memory device can separate a fault area and replace it with the substitute memory area by changing the conditions of an address decoder, thereby improving the reliability of the device.
As discussed above, if an unrecoverable read error occurs in a physical device of a conventional disk array apparatus, it is required to apply an additional load on a host apparatus to keep high reliability.